1. Field of the Invention
The present invention relates, in general, to wireless communication apparatuses and methods and, more particularly, to a wireless communication method and apparatus between multiple users, that efficiently utilizes limited wireless communication resources to increase the system capacity, and provides stable wireless communication environments so as to simultaneously transmit multiple signals to multiple users.
2. Description of the Related Art
Recently, in a wireless communication system in which a base station exchanges wireless signals with a large number of users, the increase of the capacity of wireless communication systems has become an important issue. Especially, since the next generation transmission system should be able to provide a variety of high-capacity services, such as wireless Internet and multimedia, to users existing in various states, such as mobile, nomadic and fixed states, the downlink requires a capacity higher than that of the uplink. Accordingly, the increase of the downlink capacity has become one of the most important concerns to the service providers.
Therefore, various techniques for increasing the system capacity have been suggested. One of such new techniques is an opportunistic scheduling technique, that utilizes the channel characteristics of respective users sent from a reception stage to be detected, and allows the data to be selectively transmitted using only a user channel having highest instantaneous signal-to-noise power ratio (SNR) through a scheduler provided in a transmission stage. When the opportunistic scheduling technique is utilized, the capacity of wireless communication systems can be increases in the case where a plurality of users exist in states with various dynamic ranges and speeds of channel variation. In this way, the technique of increasing the capacity using independent channel characteristics between multiple users is referred to as multi-user diversity (MUD). It is known that the MUD technique can provide a system capacity higher than that in Additive White Gaussian Noise (AWGN) channel in fading channel condition(refer to a first reference document entitled “Opportunistic beamforming using dumb antennas” by P. Viswanath, D. N. C. Tse and R. Laroia in IEEE Transaction on Information Theory, Vol. 48, No. 6, pp. 1277-1294, June 2002).
FIG. 1 illustrates a case where the above-described MUD technique using the opportunistic scheduling is applied to two users in a system using a single antenna. In FIG. 1, there is depicted a scheme of allocating communication resources to a user having the best channel quality at each instant using the channel qualities (e.g., the channel gain, SNR, rate, etc.) of respective users when the channels of different users are independently varying. A channel viewed from a transmission side is equivalent to a channel represented by a dotted line, so that this system provides an SNR higher than the average SNR of the respective users, thus having a system capacity higher than that of AWGN channel having the same average SNR.
However, if the dynamic range and speed of channel variation are low as in the case of fixed wireless access (FWA) or nomadic users, the MUD gain is significantly reduced, remarkably reducing the capacity improvement.
In order to alleviate the above problem and to maintain the advantage of the MUD technique, there has been recently proposed a random beamforming technique that compulsorily increases the dynamic range and speed of channel variation using multiple antennas, providing the diversity gain (refer to the first reference document) (a second reference document entitled “The impact of multiuser diversity in space-time block coding” by R. Goazli, R. M. Buehrer and B. D. Woerner, in IEEE Communications Letters, Vol. 7, pp. 420-424, May 2003). This technique is known more effective than a conventional multi-input multi-output (MIMO) or space-time coding (STC) technique from the aspect of capacity increase (the second reference document). Further, in the case when the correlation between the channels according to antennas is high, multiple antenna diversity techniques, such as the STC, and antenna multiplexing techniques, such as Bell Laboratories Layered Space-Time (BLAST), may not be effective. However, if the MUD technique is applied, the channels between the respective users are independent even in an environment where the correlation between the channels according to antennas is high. Thus we can obtain a gain regardless of the correlation between the channels.
FIG. 2 illustrates the configuration of a random beamforming scheme (the first reference document) comprising a multi-input single-output (MISO) system and compulsorily varying a channel by using multiple antennas, such as array antennas, in the transmitter. In FIG. 2, the transmitter output signal s(t) is multiplied by a weight vector w(t)=[w1(t), w2(t), . . . , wM(t)]T for each antenna and then transmitted in a form of x(t)=w(t)s(t), where the superscript T represents the transpose of a vector. The weight vector can be generated by the following equation [1],wm=√{square root over (αm(t))}ejθm(t), m=1, 2, . . . , M  [1]where αm(t) and θm(t) are random variables varying in the range of 0≦αm(t)≦1 and 0≦θm(t)<2π, respectively. For normalization of the total transmission power, it is assumed that
            ∑              m        =        1            M        ⁢                                                              α                              m                ⁢                                                                                        ⁡                          (              t              )                                                  2        =  1.At this time, the received signal of the k-th user passing through the channel hkH(t)=[h*1,k(t), h*2,k(t), . . . , h*M,k(t)] is expressed by the following equation [2],Yk(t)=hkH(t)w(t)s(t)+zk(t), k=1, 2, K, K  [2]where the superscript H denotes the Hermitian of a vector and zk(t) represents the noise of the k-th user that is assumed zero mean AWGN with a variance of σ2z. The channel recognized by the receiver is expressed by the following Equation [3].
                                                                                          h                  k                  *                                ⁡                                  (                  t                  )                                            =                            ⁢                                                                    h                    k                    H                                    ⁡                                      (                    t                    )                                                  ⁢                                  w                  ⁡                                      (                    t                    )                                                                                                                          =                            ⁢                                                ∑                                      m                    =                    1                                    M                                ⁢                                                                            h                                              m                        ,                        k                                            *                                        ⁡                                          (                      t                      )                                                        ⁢                                                                                    α                                                  m                          ⁢                                                                                                                                                    ⁡                                              (                        t                        )                                                                              ⁢                                      ⅇ                                                                  jθ                        m                                            ⁡                                              (                        t                        )                                                                                                                                                    [        3        ]            
If multiple antennas with random weight are used as described in the above, the dynamic range and speed of channel variation detected by the receiver can be increased, so that the capacity can be increased by the application of the above-described MUD technique.
FIG. 3 illustrates that the dynamic range of channel variation is increased by utilizing the above-described random beamforming scheme. With respect to very slowly varying channel or Rician channel, the speed and dynamic range of channel variation can be adjusted by adjusting the variation frequency of the weight of the random beam. Through this method, it was reported that the capacity can be increased when a multi-user diversity scheme is applied to fixed wireless channel or etc. (the first reference document). However, in Rayleigh fading channel having large dynamic range and high speed of channel variation, the effect of further increasing the dynamic range and speed of channel variation by the random beamforming is insignificant. Therefore, this method may not provide a large gain.
Recently, there has been an attempt to combine the random beamforming technique and MIMO singular value decomposition (SVD) based multiplexing technique together (refer to a third reference document entitled “A random beamforming technique in MIMO systems exploiting multiuser diversity” by J. Chung, C. S. Hwang, K. Kim and Y. K. Kim, in IEEE Journal on Selected Areas in Communications, Vol. 21, No. 5, pp. 848-855, June 2003). In this technique, all the mobile stations should employ multiple antennas and perform the SVD processing, requiring a large implementation complexity. Moreover, a large amount of channel information should be sent to the transmitter. If the correlation between the channels of antennas is high, the MIMO SVD technique may not be effective.
In the meantime, there has been an attempt to devise a multiplexing scheme by forming a plurality of orthogonal beams through multiple antennas while exploiting a random beamforming scheme. Thus, this method can utilize opportunistic scheduling and multi-user diversity technique (refer to the first reference document). However, this method may suffer from mutual interference between multiple beams when the number of users is small, yielding a system capacity smaller than that of a single beam scheme. Moreover, this method needs the use of multiple pilot signals as many as the number of beams.